Method to solve alignment mark blinded issues and a technology for application of semiconductor etching at a tiny area

ABSTRACT

A method of unblinding an alignment mark comprising the following steps. A substrate having a cell area and an alignment mark within an alignment area is provided. An STI trench is formed into the substrate within the cell area. A silicon oxide layer is formed over the substrate, filling the STI trench and the alignment mark. The silicon oxide layer is planarized to form a planarized STI within the STI trench and leaving silicon oxide within the alignment mark to form a blinded alignment mark. A wet chemical etchant is applied within the alignment mark area over the blinded alignment mark to at least partially remove the silicon oxide within the alignment mark. The remaining silicon oxide is removed from within the blinded alignment mark to unblind the alignment mark. A drop etcher apparatus is also disclosed.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor fabrication andmore specifically to alignment marks and semiconductor etching at tinyareas.

BACKGROUND OF THE INVENTION

Alignment marks are used to permit precise alignment ofphotolithographic masks with the wafer during masking steps to minimizemisalignment between multiple layers. However, the alignment marks are‘blinded’ after non-ODR shallow trench isolation (STI) chemicalmechanical polishing (CMP) processes. This prevents transfer of thelower alignment mark to the next, upper layer, for example a metallayer.

An additional photolithography and etching step (ODR) are required toclear out the silicon oxide residue from the ‘blinded’ alignment markfield. This increases costs, increases the cycle time and manufactureloading. Further, the ODR approach is limited by the circuit design ruleespecially as the design rule passes 0.1 μm logic and beyond.

Blind alignment marks will become a critical issue in non-OD reversetone photo/etch (ODR) processes after 0.1 μm and beyond shallow trenchisolation (STI) CMP.

U.S. Pat. No. 6,194,287 B1 to Jang et al. describes an STI process andreverse mask to clear off alignment marks.

U.S. Pat. No. 6,080,635 to Jang et al. describes discloses a method topreserve alignment marks with STI processes.

U.S. Pat. No. 6,043,133 to Jang et al. describes a process to improveSTI removal over alignment marks.

U.S. Pat. No. 6,015,744 to Tseng describes a clear out and alignmentmark process.

U.S. Pat. No. 5,188,258 to Iwashita describes a quantitative fluiddischarge device.

SUMMARY OF THE INVENTION

Accordingly, it is an object of one or more embodiments of the presentinvention to provide an improved method of unblinding alignment marks.

Other objects will appear hereinafter.

It has now been discovered that the above and other objects of thepresent invention may be accomplished in the following manner.Specifically, a substrate having a cell area and an alignment mark areais provided. The substrate having an alignment mark within the alignmentmark area. An STI trench is formed into the substrate within the cellarea. A silicon oxide layer is formed over the substrate, filling theSTI trench and the alignment mark. The silicon oxide layer is planarizedto form a planarized STI within the STI trench and leaving silicon oxidewithin the alignment mark to form a blinded alignment mark. A wetchemical etchant is applied within the alignment mark area over theblinded alignment mark to at least partially remove the silicon oxidewithin the alignment mark. The remaining silicon oxide is removed fromwithin the blinded alignment mark to unblind the alignment mark. A dropetcher apparatus is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the followingdescription taken in conjunction with the accompanying drawings in whichlike reference numerals designate similar or corresponding elements,regions and portions and in which:

FIGS. 1 to 6 schematically illustrate a preferred embodiment of thepresent invention.

FIG. 7 schematically illustrates a drop etcher used with the preferredembodiment of the present invention.

FIG. 8 is an enlarged cross-section of FIG. 7 taken at the dashed circlelabeled “FIG. 8.”

FIG. 9 is an enlarged cross-section of FIG. 7 taken at the dashed circlelabeled “FIG. 9.”

FIGS. 10a, 10 b and 10 c are respective graphs of: voltage vs. time onof the signal from the controller; pressure vs. time; and the chemicaldrop vs. time.

FIG. 11A is an enlarged schematic representation of a portion the dropetcher illustrated in FIG. 7 used with a blanket wafer monitor.

FIG. 11B is a graph of an etching profile of a blanket wafer monitor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The inventors have discovered a novel method to solve the alignment mark(AM) blinding issues that includes using a wet chemical solution topartially remove the oxide residue within the blinded alignment marksafter STI CMP processing. Furthermore, the wet chemical solution oxideresidue removal easily integrates into the subsequent silicon nitride(Si₃N₄ or SiN) strip step.

Initial Structure

As shown in FIG. 1, structure 10 is preferably a silicon substrate andincludes an overlying layer 12 lining alignment marks 16 withinalignment mark area 20. Layer 12 is preferably comprised of siliconnitride (Si₃N₄ or SiN) or polysilicon (having a thickness of from about500 to 2000 Å) and is more preferably SiN as will be used forillustrative purposes hereafter. SiN layer 12 is preferably from about100 to 2000 Å and more preferably from about 800 to 1100 Å thick.

Alignment marks 16 have a depth of preferably from about 1150 to 1250 Åand more preferably about 1200 Å. Alignment marks 16 have a width ofpreferably from about 7.8 to 8.2 μm and more preferably about 8.0 μm.

A shallow trench isolation (STI) opening 14 is formed through the SiNlayer 12 and into silicon substrate 10 within cell area 18 that isspaced apart form alignment mark area 20.

Formation of Oxide Layer 22

To form the STI, a silicon oxide (oxide) layer 22 is formed over siliconsubstrate 10, filling STI trench 14 and alignment marks 16. Alignmentmarks 16 are filled by oxide layer 22 and by the oxide residue formed bythe CMP of oxide layer 22 to form planarized STI layer 22′ (see below).

Planarization of Oxide Layer 22

Oxide layer 22 is then planarized, preferably by a chemical mechanicalpolishing (CMP) process to form planarized STI 22′ and filling (and‘blinding’) alignment marks 16 with oxide 22″,

Use of Drop Etcher 40

As shown in FIG. 4, a drop etcher 40 (such as that shown in FIG. 7) isaligned and used to dispense a wet etching chemical 25 in the form of adrop 26 only over the oxide filled 22″ alignment marks 16 withinalignment mark area 20. The planarized STI 22′ is not contacted with, oraffected by, wet etching chemical 25. The tip 24 of drop etcher 40 isaligned with the ‘blinded’ alignment marks 16 by finding water notch orby pattern recognition (align cell pattern).

Wet etching chemical 25 is preferably hydrogen fluoride (HF) or bufferedoxide etchant (BOE). If BOE is used, it has an oxide: SiN etchingsensitivity of about 10:1 with an oxide etching rate of about 1400Å/min. If HF is used, it has an oxide: SiN etching sensitivity of about15:1 with an oxide etching rate of about 9000 Å/min.

As shown in FIGS. 7 to 9, drop etcher 40 may include a control unit 30that dispenses a measured amount of wet etching chemical 25, preferablyin the form of a drop 26, from within dispenser tube 34 (having insidediameter 52) through tip 24 to wafer 10 by using pressure 50.

Line 51 connects control unit 30 to the upper portion of dispenser tube34 and through which line 51 a gas, such as preferably air or nitrogen(N₂), is pulsed through a filter 60, a pressure regulator 62 and ahigh-speed shut-off valve 64 controlled 66 by the controller 30 or atimer.

FIGS. 10a, 10 b and 10 c respectively illustrate: voltage vs. the amountof time on t_(on) of the signal 66 from the controller 30 or a timer;pressure vs. time; and the chemical drop 26 vs. time wherein:

 chemical drop 26=(t_(on)*Xr**Xp***)

* t_(on): time on of the signal 66 from controller 30 or timer

** r: shut-off valve 64 radius

*** p: gas pressure after the pressure regulator 62

with the shut-off valve 64 radius (r) being preferably from about 0.05to 5.00 mm and is more preferably from about 0.01 to 8.00 mm.

The time control is preferably from about 0.001 to 10.000 seconds andmore preferably from about 0.001 to 5.000 seconds; the pressure controlis preferably from about 0.001 to 10.000 psi and more preferably fromabout 0.001 to 5.000 psi; the tip 24 inside diameter 54 is preferablyfrom about 0.01 to 2.00 mm and more preferably from about 0.10 to 1.00mm; and the dispenser distance 70 is preferably from about 1.0 μm to 1.0cm and more preferably from about 0.5 to 6.0 mm.

As illustrated in FIGS. 8 and 9 (and see FIG. 4), upon the applicationof gas pressure 50 onto the upper surface 72 of the chemical 25 withinthe dispenser tube 34, the pressure 50 of the gas expels a portion ofthe chemical 25 through the drop etcher tip 24 to form a drop 26 as thelevel of the chemical 25 within the dispenser tube 34 is lowered to thedashed upper surface line 74.

By the precise control of the gas pressure, time and size of the tipsand nozzles involved, consistent deposits are ensured with a sped-upoutput while maintaining high quality and reduced waste. By increasingeither the gas pressure, the time duration or the tip gauge (insidediameter 54), the size of the drop and hence the etch size willincrease.

The tip 24 is preferably comprised of a non-reactive material such aspolytetrafluorethylene (PTFE), perfluoroalkoxy (PFA), or polyvinylchloride (PVC). For wet dip process optimization, the tip 24 preferablyhas: an outer diameter 56 of preferably from about 0.5 to 3.0 mm andmore preferably about 1.0 mm; and an inner diameter 54 of preferablyfrom about 0.5 to 0.7 mm and more preferably about 0.5 mm.

The tip 24 is preferably spaced from the upper surface of the structure10 a dispenser distance 70 of preferably from about 1 μm to 1 cm andmore preferably from about 0.5 mm to 6.0 mm.

In real process control, the drop 26 has an inner diameter 100 of fromabout 0.1 to 0.7 mm and an outer diameter of from about 1.0 to 3.0 mm(see FIG. 4).This ensures that the wet etching chemical drop 26 coversthe oxide 22″ filled alignment marks 16 within alignment mark area 20without impinging upon the cell area 18 and the STI 22′.

Etching of Oxide 22″ Within Alignment Marks 16

As shown in FIG. 5, the etching chemical drop 26 etches the oxide 22″within alignment marks 16 so that preferably from about 80 to 90% of theoxide 22″ within alignment marks 16 is removed and more preferably fromabout 85 to 90% in from about 10 to 100 seconds and more preferably fromabout 30 to 45 seconds.

The remainder of drop 26 is removed before the SiN strip step (seebelow).

SiN Layer 12 Strip

As shown in FIG. 6, SiN layer 12 is then stripped and removed fromsilicon substrate 10 and from within alignment marks 16 using a SiNstripping process that is preferably a wet bench clean process. Thisalso removes any remaining oxide 22″ from alignment marks 16 and any wetetching chemical 25/oxide 22 residue from silicon substrate 10. The SiNstripping process preferably uses H₃PO₄.

Thus, alignment marks 16 are now clear and transparent to the nextformed layer, such as a metal layer, so that the alignments marks 16 aretransferred to the upper, next formed layer and may be readily observedfor continued fabrication.

FIG. 11A is an enlarged schematic representation of a portion the dropetcher 24″ illustrated in FIG. 7 illustrating the inside diameter (ID)and outside diameter (OD) of an etched opening 102 shown in dashed lineafter a drop 100 from drop etcher 24″ is released from drop etcher 24″onto a monitor wafer's oxide layer 12″ over silicon substrate 10″. TheID and OD are plotted in FIG. 11B (see below).

FIG. 11B is a graph of blanket wafer monitor etch rate and etch profile(not in real application) where CPVC is chlorinated polyvinyl chlorideand the mean of BOE of 2:1 is a mixed ratio of about 40% NH₄F to about49% HF 2:1.

Advantages of the Present Invention

The advantages of one or more embodiments of the present inventioninclude:

1. saves the time and expense of an additional ODR;

2. the method is simple and inexpensive;

3. the method is easy to integrate with the SiN stripping process;

4. current dispenser technology is available for the drop technologyused in the present invention; and

5. increase STI CMP process merge.

While particular embodiments of the present invention have beenillustrated and described, it is not intended to limit the invention,except as defined by the following claims.

We claim:
 1. A method of unblinding an alignment mark, comprising thesteps of: providing a substrate having a cell area and an alignment markarea; the substrate having an alignment mark within the alignment markarea: forming an STI trench into the substrate within the cell area;forming a silicon oxide layer over the substrate, filling the STI trenchand the alignment mark; planarizing the silicon oxide layer to form aplanarized STI within the STI trench and leaving silicon oxide withinthe alignment mark to form a blinded alignment mark; applying a wetchemical etchant within the alignment mark area over the blindedalignment mark to at least partially remove the silicon oxide within thealignment mark; and removing the remaining silicon oxide from within theblinded alignment mark to unblind the alignment mark; wherein the wetchemical etchant is applied to the blinded alignment mark in the form ofa drop.
 2. The method of claim 1, including the step of forming a layerover the substrate and lining the alignment mark before the formation ofthe STI trench into the substrate.
 3. The method of claim 1, wherein thestructure is a semiconductor substrate.
 4. The method of claim 1,wherein the structure is comprised of silicon.
 5. The method of claim 1,including the step of forming a layer over the substrate and lining thealignment mark before the formation of the STI trench into thesubstrate; wherein the structure is comprised of silicon and the layeris comprised of a material selected from the group consisting of: SiNand polysilicon.
 6. The method of claim 1, including the step of forminga layer over the substrate and lining the alignment mark before theformation of the STI trench into the substrate; wherein the structure iscomprised of silicon and the layer is comprised of a material selectedfrom the group consisting of: SiN.
 7. The method of claim 1, includingthe step of forming a layer over the substrate and lining the alignmentmark before the formation of the STI trench into the substrate; whereinthe layer is comprised of SiN and is from about 100 to 2000 Å thick andthe alignment mark has a depth of from about 1150 to 1250 Å and a widthof from about 7.8 to 8.2 μm.
 8. The method of claim 1, including thestep of forming a layer over the substrate and lining the alignment markbefore the formation of the STI trench into the substrate; wherein thelayer is comprised of SiN and is from about 800 to 1100 Å thick and thealignment mark has a depth of about 1200 Å and a width of about 8.0 μm.9. The method of claim 1, wherein the silicon oxide layer is planarizedby chemical mechanical polishing.
 10. The method of claim 1, wherein thewet chemical etchant is comprised of a chemical selected from the groupconsisting of HF and BOE.
 11. The method of claim 1, wherein the wetchemical etchant is applied to the blinded alignment mark from a dropetcher.
 12. The method of claim 1, wherein the wet chemical etchant isapplied to the blinded alignment mark from a drop etcher having a tipwith an inner diameter of from about 0.5 to 0.7 mm.
 13. The method ofclaim 1, wherein from about 80 to 90% of the silicon oxide is removedfrom the blinded alignment mark by the wet chemical etchant.
 14. Themethod of claim 1, wherein from about 85 to 90% of the silicon oxide isremoved from the blinded alignment mark by the wet chemical etchant. 15.The method of claim 1, including the step of forming a layer over thesubstrate and lining the alignment mark before the formation of the STItrench into the substrate; wherein the layer is comprised of SiN and isremoved by a wet bench clean process.
 16. The method of claim 1, whereinthe wet chemical etchant is applied to the blinded alignment mark in theform of a drop from a drop etcher dispenser tube; the drop beingdispensed from the dispenser tube through the application of gaspressure.
 17. The method of claim 1, wherein the wet chemical etchant isapplied to the blinded alignment mark in the form of a drop from a dropetcher dispenser tube; the drop being dispensed from the dispenser tubethrough the application of gas pressure; the gas pressure being appliedthrough a high speed shut-off valve from a gas source.
 18. The method ofclaim 1, wherein the wet chemical etchant is applied to the blindedalignment mark in the form of a drop from a drop etcher dispenser tube;the drop being dispensed from the dispenser tube through the timedapplication of gas pressure; the gas pressure being applied through ahigh speed shut-off valve from a gas source; the shut-off valve having aradius; the amount of the drop equal to the length of the timedapplication of the gas pressure multiplied by the radius of the shut-offvalve multiplied by the amount of the gas pressure.
 19. The method ofclaim 1, wherein the wet chemical etchant is applied to the blindedalignment mark in the form of a drop from a drop etcher dispenser tube;the drop being dispensed from the dispenser tube through the timedapplication of gas pressure; the gas pressure being applied through ahigh speed shut-off valve from a gas source; the shut-off valve having aradius; the amount of the drop equal to the length of the timedapplication of the gas pressure multiplied by the radius of the shut-offvalve multiplied by the amount of the gas pressure; the length of thetimed application of the gas pressure being from about 0.001 to 5.000seconds; the radius of the shut-off valve being from about 0.05 to 5.00mm; and the amount of the gas pressure being from about 0.001 to 5.000psi.
 20. The method of claim 1, wherein the wet chemical etchant isapplied to the blinded alignment mark in the form of a drop from a dropetcher dispenser tube; the drop being dispensed from the dispenser tubethrough the timed application of gas pressure; the gas pressure beingapplied through a high speed shut-off valve from a gas source; theshut-off valve having a radius; the amount of the drop equal to thelength of the timed application of the gas pressure multiplied by theradius of the shut-off valve multiplied by the amount of the gaspressure; the length of the timed application of the gas pressure beingfrom about 0.001 to 10.000 seconds; the radius of the shut-off valvebeing from about 0.05 to 5.00 mm; and the amount of the gas pressurebeing from about 0.01 to 10.00 psi.
 21. The method of claim 1, whereinthe wet chemical etchant is applied to the blinded alignment mark in theform of a drop from a drop etcher dispenser tube; the drop beingdispensed from the dispenser tube through the application of gaspressure through a pressure regulator and a high-speed valve.
 22. Amethod of unblinding an alignment mark, comprising the steps of:providing a substrate having a cell area and an alignment mark area; thesubstrate having an alignment mark within the alignment mark area;forming an SiN layer over the substrate and lining the alignment mark;forming an STI trench through the SiN layer and into the substratewithin the cell area; forming a silicon oxide layer over the substrate,filling the STI trench and the SiN layer lined alignment mark;planarizing the silicon oxide layer to form a planarized STI within theSTI trench and leaving silicon oxide within the alignment mark to form ablinded alignment mark; applying a wet chemical etchant within thealignment mark area over the blinded alignment mark to at leastpartially remove the silicon oxide within the alignment mark; andremoving the SiN layer from over the substrate and within SiN layerlined alignment mark whereby the silicon oxide is also removed fromwithin the blinded alignment mark to unblind the alignment mark; whereinthe wet chemical etchant is applied to the blinded alignment mark in theform of a drop.
 23. The method of claim 22, wherein the substrate is asemiconductor substrate.
 24. The method of claim 22, wherein thesubstrate is comprised of silicon.
 25. The method of claim 22, whereinthe alignment mark has a depth of from about 1150 to 1250 Å and a widthof from about 7.8 to 8.2 μm.
 26. The method of claim 22, wherein the SiNlayer is from about 100 to 2000 Å thick.
 27. The method of claim 22,wherein the silicon oxide layer is planarized by chemical mechanicalpolishing.
 28. The method of claim 22, wherein the wet chemical etchantis comprised of a chemical selected from the group consisting of HF andBOE.
 29. The method of claim 22, wherein the silicon oxide layer isplanarized by chemical mechanical polishing and the wet chemical etchantis comprised of a chemical selected from the group consisting of HF andBOE.
 30. The method of claim 22, wherein the wet chemical etchant isapplied to the blinded alignment mark from a drop etcher.
 31. The methodof claim 22, wherein the wet chemical etchant is applied to the blindedalignment mark from a drop etcher having a tip with an inner diameter offrom about 0.5 to 0.7 mm.
 32. The method of claim 22, wherein from about80 to 90% of the silicon oxide is removed from the blinded alignmentmark by the wet chemical etchant.
 33. The method of claim 22, whereinfrom about 85 to 90% of the silicon oxide is removed from the blindedalignment mark by the wet chemical etchant in from about 10 to 100seconds.
 34. The method of claim 22, wherein the SiN layer and isremoved by a wet bench clean process.
 35. The method of claim 22,wherein the wet chemical etchant is applied to the blinded alignmentmark in the form of a drop from a drop etcher dispenser tube; the dropbeing dispensed from the dispenser tube through the application of gaspressure.
 36. The method of claim 22, wherein the wet chemical etchantis applied to the blinded alignment mark in the form of a drop from adrop etcher dispenser tube; the drop being dispensed from the dispensertube through the application of gas pressure; the gas pressure beingapplied through a high speed shut-off valve from a gas source.
 37. Themethod of claim 22, wherein the wet chemical etchant is applied to theblinded alignment mark in the form of a drop from a drop etcherdispenser tube; the drop being dispensed from the dispenser tube throughthe timed application of gas pressure; the gas pressure being appliedthrough a high speed shut-off valve from a gas source; the shut-offvalve having a radius; the amount of the drop equal to the length of thetimed application of the gas pressure multiplied by the radius of theshut-off valve multiplied by the amount of the gas pressure.
 38. Themethod of claim 22, wherein the wet chemical etchant is applied to theblinded alignment mark in the form of a drop from a drop etcherdispenser tube; the drop being dispensed from the dispenser tube throughthe timed application of gas pressure; the gas pressure being appliedthrough a high speed shut-off valve from a gas source; the shut-offvalve having a radius; the amount of the drop equal to the length of thetimed application of the gas pressure multiplied by the radius of theshut-off valve multiplied by the amount of the gas pressure; the lengthof the timed application of the gas pressure being from about 0.001 to5.000 seconds; the radius of the shut-off valve being from about 0.05 to5.00 mm; and the amount of the gas pressure being from about 0.001 to5.000 psi.
 39. The method of claim 22, wherein the wet chemical etchantis applied to the blinded alignment mark in the form of a drop from adrop etcher dispenser tube; the drop being dispensed from the dispensertube through the timed application of gas pressure; the gas pressurebeing applied through a high speed shut-off valve from a gas source; theshut-off valve having a radius; the amount of the drop equal to thelength of the timed application of the gas pressure multiplied by theradius of the shut-off valve multiplied by the amount of the gaspressure; the length of the timed application of the gas pressure beingfrom about 0.001 to 10.000 seconds; the radius of the shut-off valvebeing from about 0.05 to 5.00 mm; and the amount of the gas pressurebeing from about 0.001 to 10.000 psi.
 40. The method of claim 22,wherein the wet chemical etchant is applied to the blinded alignmentmark in the form of a drop from a drop etcher dispenser tube; the dropbeing dispensed from the dispenser tube through the application of gaspressure through a pressure regulator and a high-speed valve.